A magnetic memory (hereinafter also referred to as a magnetoresistive random access memory (MRAM)) is a nonvolatile memory capable of high-speed operation. Therefore, MRAMs are expected to serve as novel nonvolatile work memories, and are being developed. In an MRAM, a magnetic tunnel junction (MTJ) element is used as the storage element. This MTJ element includes a first magnetic layer, a second magnetic layer, and a nonmagnetic insulating layer disposed between the first magnetic layer and the second magnetic layer. One of the first and second magnetic layers has a fixed magnetization direction and is also called the reference layer, and the other one of the first and second magnetic layers has a changeable magnetization direction and is also called the storage layer. The electrical resistance of this MTJ element is low when the magnetization directions of the reference layer and the storage layer are parallel to each other, and is high when the magnetization directions are antiparallel to each other.
In an MRAM, information “0” corresponds to one of a state where the magnetization directions of the reference layer and the storage layer are parallel to each other and a state where the magnetization directions are antiparallel to each other, and information “1” corresponds to the other one of the states. It is possible to determine whether the magnetization directions of the reference layer and the storage layer are parallel or whether the magnetization directions are antiparallel, using a magnetoresistive effect.
Writing into the MTJ element is performed by switching the magnetization direction of the storage layer. One of the known techniques for such writing is spin transfer torque magnetization switching (hereinafter also referred to as spin transfer torque (STT)). Writing by this STT is performed by applying current between the reference layer and the storage layer via the nonmagnetic insulating layer, and therefore, the nonmagnetic insulating layer might be broken at a time of writing. Current is also applied to the MTJ element at a time of information (data) reading. Therefore, read disturb might occur, as the magnetization direction of the storage layer is reversed by STT when data is read out.
Another one of the known techniques for writing is a technique using a spin Hall effect or a spin-orbit interaction (spin-orbit coupling). A spin-orbit interaction is a phenomenon in which current is applied to a nonmagnetic layer so that electrons having spin angular momenta (hereinafter also referred to simply as the spin) of the opposite orientations from each other are scattered in the opposite directions, and a spin current Is is generated. At this point, the spin s, the spin current Is, and the electron current Ie (of the opposite direction from the current) satisfy the relationship:
Is∝s×Ie That is, the spin current Is is proportional to the outer product of the spin s and the electron current Ie. As an MTJ element is stacked on the nonmagnetic layer, spin orbit torque (SOT) is applied to the storage layer of the MTJ element by virtue of the spin current generated in the nonmagnetic layer, and the magnetization direction of the storage layer can be reversed. As the polarity (direction) of the current flowing in the nonmagnetic layer is reversed, the spin orbit torque (SOT) being applied to the storage layer of the MTJ element is also reversed. That is, the magnetization direction of the storage layer can be switched to a direction parallel or antiparallel to the magnetization direction of the reference layer by controlling the current to be applied to the nonmagnetic layer. An MRAM that performs writing by using this principle is called an SOT-MRAM.
An SOT cell that is a memory cell used in an SOT-MRAM has a memory element with three terminals. Since a read current path and a write current path are different, two or three transistors are provided for one SOT cell. Therefore, the area occupied by memory cells becomes larger.